Journal of the American Chemical Society, Vol.119, No.1, 53-58, 1997
Shapes of Dendrimers from Rotational-Echo Double-Resonance NMR
The solid-state shape, size, and intermolecular packing of a fifth-generation dendritic macromolecule were determined by a combination of site-specific stable-isotope-labeling, rotational-echo double-resonance (REDOR) NMR and distance-constrained molecular dynamics simulations. REDOR experiments measured dipolar couplings between C-13 atoms located near the chain ends and an F-19 label placed at the core of benzyl ether dendrimers (generations 1-5) based on 3,5-dihydroxybenzyl alcohol as the monomeric repeat unit. Intramolecular C-13-F-19 coupling was distinguished from intermolecular coupling by dilution with nonlabeled dendrimer. The average intramolecular C-13-F-19 distances for generations 3-5 were each approximately 12 Angstrom, which indicates inward-folding of chain ends with increasing generation number. The average intermolecular C-13-F-19 dipolar coupling decreased with increasing generation number, consistent with decreased interpenetration for larger dendrimers. The measured intra- and intermolecular distances for the fifth-generation dendrimer were used as constraints on energy minimizations and molecular dynamics simulations, which resulted in visualizations of the dendrimer packing and an estimate of density in the solid state.
Keywords:DENDRITIC MACROMOLECULES;STARBURST DENDRIMERS;BEHAVIOR;CHEMISTRY;VISCOSITY;MOLECULES;POLYMERS